1. Field of the Invention:
The present invention relates to the preparation of 1,6-hexenedioic acid diesters. Such diesters can be hydrogenated into the corresponding adipic acid diesters, or adipates, which can in turn be hydrolyzed to form adipic acid. Adipic acid, one of the principal raw materials for nylon 66, is today produced in vast amounts.
More especially, the present invention relates to the preparation of 3-hexenedioic acid diesters by reacting carbon monoxide and an alcohol with a dichlorobutene in the presence of a palladium-based catalyst.
2. Description of the Prior Art:
It is known to this art (cf. Journal of the American Chemical Society, 86, p. 4,351 (1964)) that ethyl vinylacetate may be prepared by reacting, at 120.degree. C. under a pressure of approximately 130 atm, carbon monoxide, ethanol and allyl chloride, in the presence of palladium (II) chloride.
It is also well known to this art (cf. Journal of Organometallic Chemistry, 188, p. 229 (1980)) that methyl vinylacetate may be produced by reacting, at 80.degree. C. under 200 atm, carbon monoxide, methanol and allyl chloride, in the presence of a catalyst based on bis(triphenylphosphine)palladium (II) chloride and tin (II) chloride in methyl isobutyl ketone or benzene.
However, simply extrapolating these techniques to a substrate such as 1,4-dichloro-2-butene or 3,4-dichloro-1- butene does not provide the desired diesters.
Moreover, a process in which 1,4-diethoxy-2-butene is reacted with carbon monoxide and ethanol in the presence of palladium (II) acetylacetonate and hydrochloric acid, at a temperature of 160.C, the initial pressure of carbon monoxide being 150 kg/cm.sup.2, to produce, in particular, the diethyl ester of 3-hexenedioic acid, is described in the Japanese patent application published under No. 68/23,929. However, the selectivity for the 3-hexenedioate remains low, despite relatively severe conditions of temperature, pressure and reaction time (cf. the detailed study by the same authors in Tetrahedron, 25, p. 4,189-4,190 (1969)).
Ongoing investigations with a view towards the development of novel techniques for the preparation of adipic acid have demonstrated that serious need exists for an effective method for the preparation of linear diesters of 3-hexenedioic acid, referred to simply as hexene-1,6-dioates, which, after hydrogenation, provide the corresponding adipate. Equally seriously need exists for effective means for the preparation of hexene-1,6-dioates from dichlorobutene, and more particularly from 1,4-dichloro-2-butene, 3,4-dichloro-1-butene or mixtures thereof in all proportions.
Moreover, it too is known to the art that the chlorination of butadiene in the gaseous phase, for example in the presence of catalysts based on palladium and copper, results in the facile and selective formation of a mixture of 1,4-dichloro-2-butene and 1,2-dichloro-3-butene (cf. U.S. Pat. No. 3,823,096).